I think the sprinting analogy is a good one. I think the biggest piece to take into account is that the harder you sprint, the longer you need to rest. I think just like a good coach, a PM needs to play with the sprint/rest/slow cycles until they get to a cadence that pushes, but does not hurt the team. Also, as with running, as the team adapts, the cadence can and should change to better suit their progress.